Currently, as one of GNSSs (Global Navigation Satellite Systems), there is GPS (Global Positioning System).
In GPS, positioning of a receiver is performed by receiving GPS signals transmitted from a plurality of GPS satellites, and using code phases and carrier phases of the received GPS signals. In GPS, a different diffusion code is set for every GPS satellite, and each GPS signal is code-modulated by the different diffusion code.
In such GPS, the GPS signal from a GPS satellite different from a target GPS satellite may be mistakenly captured as a target GPS signal and tracked. Such a phenomenon is called cross-correlation.
As a method of preventing cross-correlation, in Patent Document 1, in code phases with a single search frequency, by comparing integrated correlation values at respective code phase points, whether the acquired integrated correlation values are obtained from the target GPS signal or from cross-correlation is determined. FIG. 1 is a chart illustrating a concept of the signal searching method described in Patent Document 1.
With the method of Patent Document 1, an integrated correlation value is acquired at each code phase with a single search frequency, and these values are compared to each other. With the method of Patent Document 1, an integrated correlation value Vpk1 that is the highest value among the integrated correlation values at all the code phases is detected. Next, with the method of Patent Document 1, an integrated correlation value Vpk2 that is the second highest value among the integrated correlation values at all the code phases is detected. With the method of Patent Document 1, a ratio between the integrated correlation value Vpk1 which is the highest value and the integrated correlation value Vpk2 which is the second highest value is calculated. With the method of Patent Document 1, if the ratio is higher than a predetermined threshold, a code phase Cpk1 corresponding to the integrated correlation value Vpk1 which is the highest value is considered to be produced by the target signal, and the signal of the code phase Cpk1 is captured. On the other hand, if the ratio is lower than the threshold, the signal search continues.
Moreover, in Patent Document 2, the following method is used. First, a strong signal is detected based on a result of calculating an integrated correlation value in a fixed integrating period of time at a predetermined frequency interval. By utilizing that a difference between a frequency of the detected strong signal and a frequency of a capturing target has a predetermined connection with a signal level, a detection of cross-correlation is performed. Then, when the detected strong signal is determined to be cross-correlation, the signal search continues.
As described above, with the conventional signal searching methods, an integrating period of time for each code phase and frequency is fixed until the search can be carried out over a searching range, in other words, an entire code phase range and an entire frequency range in which the signal search is performed. Then, if the target signal is not successfully detected after such an entire search is completed at least once (a plurality of times depending on the case), the integrating time period is extended and the signal search is performed.
Moreover, in a case of detecting a weak signal, the integrating time period is set to be longer than the integrating time period for strong signal detection.